(1) Field of the Invention
This invention generally relates to an isotopically altered optical fiber, and is specifically concerned with a silica fiber either depleted of or enriched with Si-29 isotope atoms, or depleted of or enriched with Ge-73 isotope atoms.
(2) Description of the Related Art
Light travels down a fiber-optic cable by bouncing repeatedly off the walls. Each photon bounces backwards and forwards from one wall to the other down the cable. If the light hits the wall at a really shallow angle (less than 42 degrees), it reflects back in again—as though the wall were really a mirror. This phenomenon is called total internal reflection and is one of the things that keeps light inside the cable.
The other thing that keeps light in the cable is the structure of the cable, which is made up of two separate parts. The main part of the cable—in the middle—is called the core and that is the part the light travels through. Wrapped around the outside of the core is another layer of called the cladding. The cladding keeps the light inside the core. It does this because it is has a lower refractive index than the core. This causes total internal reflection that stops the light escaping and keeps it bouncing down the core.
Optical loss is a limiting factor in the design and construction of optical networks and links, which typically include hundreds of kilometers of silica-based optical fiber. Optical losses in silica fibers are predominantly caused by two factors: (1) Rayleigh scattering, which falls off as a function of 1/λ4 (where λ is wavelength) and which dominates for shorter wavelengths; and (2) infrared absorption by the silica, which dominates for longer wavelengths. Typical Germania (GeO2) doped silica-core fibers have losses of 0.189 db/km to 0.200 db/km. between 1510 nm and 1610 nm.
There was a previous attempt to develop an optical fiber with lower transmissivity losses by means of isotopically altering the fiber regions. See U.S. Pat. Nos. 6,810,197 and 6,870,999. The improvement in loss was limited to about 0.145 to 0.155 db/km, and was accomplished primarily by shifting the wavelength of minimum optical loss to about 1670 nm, and partly by changing the index-of-refraction dopant from Germania to Oxygen-17, although the inventors may not have recognized the reason for the reduction in loss due to their use of Oxygen-17.
Fiber-optic scientists and engineers have not recognized that Si-29 isotope is the source of nearly all of the variation of index of refraction of silica from 1.0000, with natural-isotope-proportion Oxygen-17 providing a minor increase at normal (natural) levels. Likewise, such scientists and engineers have not recognized that the Ge-73 isotope (the dopant which, is normally used) increases the index of refraction of fused silica in its natural isotopic proportions, from 1.46 to about 1.47. It is also not recognized that it is the Si-29 dopant which is responsible for the large majority of Rayleigh scattering present in existing-technology optical waveguides. They do not the Si-29 as a dopant since is a naturally occurring stable isotope of silicon.
Thus, a reduction in Si-29 isotopic proportion in silica of, say, a factor of 100 (from nature's 4.67% atom/atom to 0.0467% atom/atom) will result in a material with an index of refraction of 1.005, and a reduction by a factor of 33 will result in a material with an index of refraction of 1.015. These two materials, with a difference in index of refraction of 1.015−1.005=0.010, are well within the right range to become the cladding, and core, respectively, of a new fiber.
Similarly, U.S. Pat. No. 6,490,399 describes the substitution of silicon-30 for silicon-28 isotope, which has a similar effect of moving to the right of the graph the intrinsic IR absorption line. This results in opening up a new region of useable transmission. See FIG. 2 showing a region labeled “B” from about 1610 through about 1710 nanometers wavelength, for the substitution of both Si-30 for Si-28, and O-18 for O-16.
U.S. Pat. No. 6,810,197, in its “Summary of the Invention” (column 1, line 58 to column 3, line 14) describes a reduction in the number of needed amplification stations for a cross-Atlantic link, by 11 units, due to an increase in the possible inter-amplifier spacing from 125 kilometers to 156 kilometers. This benefit is almost certain to be illusory in practice, however. Any practical link that transmits from 1610 through 1710 will also be designed to employ the 1510-1610 band, and the isotopic substitution will not appreciably assist the fiber's transmission in most of the 1510-1610 nm band. Since the same amplifier station that amplifies the 1610-1710 band will also be the one to amplify the 1510-1610 band, proper operation on the latter band will require maintenance of the same 125 kilometer inter-amplifier spacing as is currently required. Thus, the only useable improvement will be a broadening of the useable bandwidth on which signals may be sent. In other words, substitution of Si-30 for Si-28 and O-18 for O-16 does not actually enable any savings due to reduction in waveguide loss, and even the widened band (including the 1610-1710 region) is likely to be beneficial only in links in which wavelength division multiplex (WDM) signals already occupy all of the 1510-1610 bandwidth.
Similarly, the reference in U.S. Pat. No. 6,490,399 to Patent Abstracts of Japan, JP-A-60090845, describes a method of employing a deuterium rinse of the porous SiO2 preform to replace existing —OH groups with —OD groups, thus shifting their absorption bands (including 1400 nm) to much longer wavelengths—longer than 1710 nm. Yet, that technology is described as “costly”, in part because fiber manufacturers have already done a good job of reducing —OH content, in part by continual improvement of the C12 treatment which was described in U.S. Pat. No. 3,933,454, columns 7, line 1 to 8, line 68.
However, this merely means that the —OH absorption spectrum, especially at 1400 nm, see FIG. 2, is sufficiently low compared to the “Rayleigh Scattering” minimum line, see FIG. 3, so as to make further improvement seemingly without benefit. The instant invention, by means (in part) of reducing Si-29 concentrations by large amounts, up to a factor of 50-100 or more, has the effect of greatly reducing the altitude of the “Rayleigh Scattering” minimum by a large, related amount, which will enable extra utility for a deuterium (D2) rinse as was described in
JP-A-60090845.
Therefore, an embodiment of the instant Si-29-reducing invention will likely have further unanticipated benefits from both a deuterium (D2) rinse, as well as a substitution of Si-30 for Si-28, or a substitution of O-18 for O-16, or both. A full implementation of these modifications may result in an optical waveguide that has a transmission bandwidth from at least 1230 nm to about 2000 nm at a loss of about 0.01 db/km or lower, and so can achieve trans-Atlantic transmission with no or, at most, one amplifier station.
This attempt shifted the line representing “IR Absorption” (see FIGS. 4 and 5) to the right on these graphs. This had the effect of reducing the minimum absorption, caused by the sum of “IR absorption”, “Rayleigh [scattering]”, and “UV” to be slightly lowered, with the resulting transmission bandwidth somewhat broadened.
The instant invention, however, attacks not only the “IR Absorption” line, but in fact also the “Rayleigh Scattering” line. See FIG. 10, long-hatched line, labeled “Rayleigh scattering.”
A reduction in the quantity of Si-29 scattering centers by a factor of “X” will reduce the altitude of the “Rayleigh scattering” line by a factor within the range of “X” and the square-root of “X”. This will result, for a factor of 33 reduction, in Si-29 concentration, in between a factor of 33 and the square root of 33 (about 5.9) reduction in attenuation due to Rayleigh scattering effects. This dramatically reduces the overall attenuation seen in the 1310 nm band, as well as in wavelengths up to approximately 1650 nanometer.
Of course, a combination of a portion of the features of U.S. Pat. No. 6,810,197, the substitution of
O-18 for O-16 in both the core and regions of the cladding near the core, and the dramatic reduction in Si-29 isotope proportion down to and through a factor of 100 reduction (to 0.0467% of Si-29 atom/atom) combines to cause a remarkable reduction in overall attenuation due to the signal's passage through the optical waveguide.
The authors of U.S. Pat. No. 6,810,197 were under the impression that at least some O-17 isotope was necessary to provide useful benefits, co-substituting with Oxygen-18 for the majority of Oxygen-16 that would ordinarily be present in non-isotopically-modified fiber. See, for examples, claims 1, 3, 4, and 9 in U.S. Pat. No. 6,810,197.
In contrast, this instant invention specifies the presence or absence of an O-17 isotope, but in proportions sufficiently lower than to overlap the claims of U.S. Pat. No. 6,810,197 or other patents or applications.
Persons familiar with the art of fiber optic waveguide design, i.e. fiber optic scientists and engineers, will be able to define, for a given transmission wavelength and core diameter, the necessary index of refraction difference for it to function as a single-mode transmission medium, or alternatively as a multi-mode transmission medium.
The fused silica index of refraction may be adjusted from the natural-isotope-distribution value of about 1.46, to virtually any value down to 1.0000, depending on the reduction in the proportion of Si-29 isotope achieved. So, the example above of the indices of refraction being 1.015 and 1.005 is exemplary and without limitation.
U.S. Pat. No. 6,128,928 describes the anti-free-oxygen benefits of a small doping of germanium oxide added to the cladding or inner-cladding layer of an optical fiber. In that context, however, the index-of-refraction-raising effect of the Ge-73 isotope (which the author of U.S. Pat. No. 6,128,928 does not recognize as the very large majority of the source of the index-of-refraction-raising-effect) is a drawback. The inventor of the instant invention, instead, specifies the addition of ONLY (or a large majority) of Germanium atoms other than Ge-73 isotope atoms, in order to obtain the same benefit of U.S. Pat. No. 6,128,928 without raising the index of refraction. The author of the U.S. Pat. No. 6,128,928 patent clearly did not anticipate the possibility that an isotopically-modified sample of germanium oxide could be used, instead of a natural-isotope sample.
The Inventor of the instant invention was familiar with the principles of optics and of waveguides, having taken a Physics course numbered ‘8.03’ at MIT in the fall of 1977.
In early 2007, the Inventor had the opportunity to read a 1979 book on the very highly technical aspects of optical-fiber construction and use. In November/December 2008, the Inventor had the opportunity to read Corning Glass Works v. Sumitomo Electric U.S.A., both the district court case at 671 F. Supp 1369 (S.D.N.Y. 1987) and the Federal Circuit appeal case at 868 F.2d 1251 (Fed. Cir. 1989). This provided a very extensive discussion of the history of optical-fiber research and details as to its construction and design.
The Inventor also happened to obtain a list of naturally-occurring isotopes of each element, about 256 in total. Silicon consists of about 92% Si-28, 4.67% Si-29, and 3.1% Si-30. Germanium is about 7.8% Ge-73. A given nucleide (an isotope's nucleus) possesses a property called ‘spin’ (actually, ‘electromagnetic spin’) if it had either an odd number of protons or an odd number of neutrons. Thus, of silicon's isotopes, only the Si-29 (4.67% atom/atom) had ‘spin’, and only the Ge-73 (7.8% atom/atom) had ‘spin’.
‘Spin’ may be thought of as a permanent wobble caused by the fact that there remains a single, unpaired nucleon present. It causes a vibration of the (positively charged) nucleus, making that nucleus behave something like a tiny bar-magnet. This spin could is used in Nuclear Magnetic Resonance analysis, most commonly with Hydrogen-1 atoms, and in Magnetic Resonance Imaging. Isotopes are also used, occasionally, as ‘tracers’, to follow the mechanism of chemical reactions.
By reading the Corning case, the Inventor knew that the addition of about 8% (weight/weight) of Germania (Ge02) to silica (Si02) had the effect of raising the index of refraction of pure silica (at 1.4584) to about 1.466. But why, the Inventor wondered, did it do so? It occurred to me that since the silicon atoms were only 4.67% spin-containing, and the replacement germanium atoms were 7.8% Ge-73, the Inventor thought that maybe the presence of electromagnetic spin-containing atoms was the underlying reason that materials even have an index of refraction greater than that of air or a vacuum (1.000), and it turns out that the Inventor was right. Even then, the Inventor understood that the Inventor did not know if a given Ge-73 atom had a greater effect, overall, on index of refraction than a Si-29 atom, but that was a question that the Inventor could not then answer.
But, the realization that Si-29 may be the underlying reason that silica has an index of refraction over 1.000 led to a number of ideas in quick succession:
1. You could add Si-29 to silica, rather than adding Ge02 to silica, to increase its index of refraction over that of the cladding layer.
2. You could decrease the proportion of Si-29 isotope atoms in the cladding layer, rather than increasing them in the core, and thus produce the differential in index of refraction necessary to have a functioning optical waveguide.
Either of these ideas is interesting, but they would only have provided a small increase in benefit for the optical-fiber manufacturing industry. Either idea would slightly reduce the optical loss over Germania-doped optical fibers, but in both cases the velocity factor would remain close to the 68% of c characteristic of existing optical fibers.
The big question was: how low could the index of refraction of the core and cladding be brought down and the core and cladding still function as a waveguide? As far as is known, the only limit was that it was impossible to lower the index of refraction of the cladding material to a value of 1.0000, the same value as a vacuum. And, with such a cladding, the core would probably have to have an index of refraction about 0.008 larger, and thus it would be 1.0080.
The resulting fiber would have a velocity factor of 1/1.008, or 99.2% of c. The Inventor realized that would be very valuable for users of fiber optics to be able to accelerate the signals from the existing velocity factor of 0.68 to near 0.99. The Inventor was not aware of the invention of an optical fiber with a velocity factor of 98-99% of c.
But this was not surprising since there is little need for isotopes of various elements and thus science and industry only rarely attempt to separate isotopes of the same element.
In the field of Chemistry, tracer (stable) isotope-tagged chemicals are sometimes used to analyze chemical reactions
Three patents were granted in the early-2000 time frame, one to Deutsche Telekom and two to Corning, on the very subject of stable-isotope enhancement. But the only isotopic ratios they were talking about modifying were Si-28 versus Si-30, or 0-16 versus 0-18, and to a smaller extent 0-16 versus 0-17. Si-29 simply was not considered.
The mechanism to generate the necessary isotopically-modified precursor (SiC14; silicon tetrachloride) already exists. See the “Silicon Kilogram Project” (Google “Silicon Kilogram Sphere”). They separated the silicon-containing precursor (which was probably either silane (SiH4) or silicon tetrafluoride (SiF4)), in Russian gas centrifuges, and converted it into single-crystal silicon later. Instead, the instant invention requires the silane or silicon tetrafluoride turned into SiC14, which can be directly used as input to the optical-fiber manufacturing process of the same type that Corning patented in 1976.
Development of an optically transmissive material and a waveguide which has much reduced index of refraction represents a great improvement in the optical arts and satisfies a long felt need of the optical engineers.